Manhole explosions are rare and result from various factors, but they can occur without warning. The energy released in a major manhole explosion can lift a 200-pound cast-iron cover from its frame, in some cases causing it to become airborne and posing a hazard to both life and property.
EPRI conducted the research in collaboration with two Michigan companies, Detroit Edison and Stabiloc of Warren. Stabiloc developed a controlled pressure relief mechanism, which was tested in a simulated working environment.
The cover is designed to remain engaged to the manhole frame in all but the most severe explosions. It uses two latches: a fixed latch and an adjustable breakaway latch equipped with two shear pins.
In a minor explosion, the mechanism allows the cover to rise about two inches to relieve pressure and then fall back into its frame. In a more powerful explosion, the primary pin may shear, allowing the cover to rise another inch or so. In a major explosion, the secondary pin may shear to release the cover from its frame.
EPRIs testing confirmed that the mechanism can restrain the cover while allowing it to rise slightly to release internal pressure, said Matt Olearczyk, manager of distribution research for EPRI. The research team also determined the necessary pin sizes to provide controlled pressure relief and prevent roadbed damage for minor and moderate explosions.
Detroit Edison has installed about 1,200 Stabiloc covers to enhance safety.
This project was a landmark collaborative effort, said Vince Dow, Detroit Edison vice president, distribution operation. The project produced an effective, elegant solution that improves worker and public safety.
Nirmal Singh, a dielectric scientist, was Detroit Edisons lead researcher on the project. He commented that Detroit Edison has been an EPRI member for 30 years, and, this project stands out as a major benefit of our membership and a major benefit to our company.
EPRI performed the analysis at its High-Voltage Laboratory facility in Lenox, Massachusetts.
EPRI and Detroit Edison researchers used a series of explosions to evaluate the performance of covers with and without the controlled pressure relief mechanism, and with various sizes of shear pins in the locking mechanism. Test results provided performance data to help the teams engineers to optimize the covers design.